1. Field of the Invention
The present invention generally relates to a spectrum analyzing system for receiving and sweeping a measuring signal to analyze a spectrum thereof and, more particularly, to a spectrum analyzer for discretely selecting and receiving a frequency component included in a burst signal and measuring the frequency component.
2. Description or the Related Art
A conventional spectrum analyzer has an arrangement of FIG. 17 to display a frequency component included in a measuring signal as a series of spectral waveforms on a screen.
Referring to FIG. 17, reference numeral 1 denotes a measuring unit for sweeping and detecting a measuring signal. The measuring unit 1 is arranged to cause a mixer 3 to frequency-convert the input measuring signal in accordance with a local oscillation signal from a sweep oscillator 2 and cause a detector 5 to detect a signal passing through a band-pass filter 4.
Reference numeral 7 denotes a sweep control circuit for sweeping and controlling the frequency of the local oscillation signal in accordance with a sweep signal.
Reference numeral 8 denotes a display unit for displaying a detection output from the detector 5 on a frequency axis. A spectral waveform of the measuring signal falling within the frequency range determined by the sweep signal is displayed on the screen of the display unit 8.
For example, when a measuring signal having a frequency F.sub.1 shown in FIG. 18A is input, assuming that a sweep signal corresponding to the frequency range from a start frequency F.sub.0 lower than the frequency F.sub.1 to a frequency several times the frequency F.sub.1 is output, as shown in FIG. 18B, a spectral waveform shown in FIG. 18C is displayed on the screen of the display unit 8.
When a burst signal of the frequency F.sub.1 which is pulse-modulated with a period shorter than a sweep period of the sweep signal (as shown in (b) of FIG. 19) is, however, input, as shown in (a) of FIG. 19, the spectrum of a nonsignaling period T.sub.0 contains only noise components, as shown in (c) of FIG. 19. As a result, the spectrum of the measuring signal present within the range of the reception frequency swept during the non-signaling period T.sub.0 cannot be observed.
In order to eliminate the unobservable range, the sweep period may be shortened. However, since the band of the band-pass filter 4 and the response time of the detector 5 are limited, the conventional spectrum analyzer cannot cope with a high-speed burst modulated signal.
In summary, a general spectrum analyzer comprises a local oscillator and a mixer. The local oscillator is frequency-swept for a predetermined sweep period to sequentially convert signals in a frequency range to be measured into intermediate frequency (IF) signals. The bandwidth of the received IF signals are narrowed by a band-pass filter having a desired bandwidth. The frequency-analyzed signals are then detected. The swept and analyzed signal is displayed such that its frequency is plotted along the abscissa and the corresponding signal amplitude is plotted along the ordinate.
When the above general spectrum analyzer is used to perform spectrum analysis upon reception of a burst signal (e.g., a signal obtained such that a high-frequency signal F.sub.1 amplitude-modulated by a rectangular low-frequency signal F.sub.0 used as in a television system repeats ON and OFF intervals at a period of the low-frequency signal, as shown in FIG. 20), the sweep time of the spectrum analyzer is not sufficiently matched with the ON interval (or the OFF interval) of the burst signal. As a result, a satisfactory measurement result cannot be obtained.
The spectrum of the burst signal shown in FIG. 20 has a well-known distribution, as shown in FIG. 21. This distribution is measured and observed with the general spectrum analyzer for performing so-called continuous sweep for a sweep time long enough to render the ON/OFF intervals of the burst signal negligible.
When the sweep time is shortened in this general spectrum analyzer, the ON/OFF intervals of the burst signals pose a serious problem, as shown in FIG. 22.
That is to say, FIG. 22 is an example of a measurement of a harmonic wave of a signal F.sub.1 included in the burst signal, as shown in (a) of FIG. 22. As shown in (b) of FIG. 22, the frequency of a local oscillator is swept to receive signals of the measuring frequency F.sub.1 to frequency F.sub.4 by a sweep voltage (the frequency of the local oscillator is a frequency as a sum of the measuring frequency and an intermediate frequency F.sub.if). Since the burst signal of the input signal is set in a non-signaling state during the OFF interval T.sub.0, the measuring frequencies F.sub.1 and 3F.sub.1 received in this state cannot be measured. The measuring frequencies 2F.sub.1 and 4F.sub.1 can be measured because the burst signal is present during the reception time.
Strong demand has arisen for a spectrum analyzer compatible with a burst signal in accordance with recent developments in the field of communications.
Prior art for analyzing high-frequency signals included in burst signals by improving conventional spectrum analyzers are exemplified by Published Unexamined Japanese Patent Application No. 63-132178 (to be referred to as Reference 1 hereinafter), Published Unexamined Japanese Utility Model Application No. 62-189669 (to be referred to as Reference 2 hereinafter), and Published Unexamined Japanese Patent Application No. 2-231576 (to be referred to as Reference 3 hereinafter). These references will be described below.